Plasma torches, also known as electric arc torches, are commonly used for cutting and welding metal workpieces by directing a plasma consisting of ionized gas particles toward the workpiece. In a typical plasma torch, a gas to be ionized is supplied to the front end of the torch and flows past an electrode before exiting through an orifice in the torch tip. The electrode, which is a consumable part, has a relatively negative potential and operates as a cathode. The torch tip is adjacent to the end of the electrode at the front end of the torch and constitutes a relatively positive potential anode. When a sufficiently high voltage is applied to the electrode, an arc is caused to jump the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as a flame that extends externally off the tip. As the torch head or front end is brought down towards the workpiece, the arc jumps or transfers between the electrode and the workpiece because the impedance of the workpiece to ground is lower than the impedance of the torch tip to ground. During this “transferred arc” operation, the workpiece itself serves as the anode.
In a conventional plasma torch, an electrode having external threads engages an internally threaded bore in the cathode body to secure the electrode to the torch head. However, it is expensive to perform a threading operation on consumable items such as electrodes. Furthermore, a threaded electrode is prone to errors in centering the electrode on the axis of the plasma torch. Consequently, there is a need for a less expensive electrode-cathode locking assembly which effectively centers the electrode on the axis of the plasma torch.
A number of conventional torches provide both a plasma (i.e. primary) gas flow volume and a secondary (e.g., cooling) gas flow volume. The ratio of plasma gas flow volume to secondary gas flow volume is adjusted by replacing the tip assembly with a different tip assembly having flow passaging sized to provide the desired ratio. In some existing torches, a first gas supply provides the plasma gas (e.g., nitrogen or oxygen) and a second gas supply provides the secondary gas (e.g., a separate supply of nitrogen or oxygen). Alternatively, a secondary fluid such as water may be provided to cool the tip. In any event, supplying two separate fluids within the same torch increases the cost of manufacturing and operating the torch.
Other conventional torches use the same supply of gas for both plasma gas and secondary gas. However, these torches have a multiple-piece tip assembly construction. Thus, replacing the tip assembly to adjust the ratio of plasma gas flow volume to secondary gas flow volume is cumbersome and time-consuming because it requires the operator to replace a plurality of items.
Existing plasma torches may be found in both “non-contact start” and “contact start” varieties. In non-contact start torches, the tip and electrode are typically maintained at a fixed physical separation in the torch head. When a high frequency high voltage is applied to the electrode (relative to the tip), a pilot arc is established therebetween. As mentioned above, when the torch head is moved toward the workpiece, the arc transfers to the workpiece. Among the disadvantages of non-contact start torches is the expense of the additional circuitry required to generate the pilot arc. These torches may also produce large amounts of high frequency, high voltage electromagnetic waves that can cause electrical interference with other electrical equipment in the area.
By way of contrast, in conventional contact start torches the tip and/or electrode move axially relative to each other along a longitudinal axis of the electrode. For example, the tip may be biased by a spring such that a clearance distance is maintained between the tip and electrode. To initiate a pilot arc, the torch operator places the torch head in contact with the workpiece with sufficient force to cause the forwardly-biased tip to be pushed in a rearward direction relative to the electrode. By compressing the biasing spring and allowing the tip and electrode to make electrical contact, the operator establishes the pilot arc. As the operator moves the torch head away from the workpiece, the tip moves forwardly away from the electrode under the bias of the spring which generates the pilot arc and transfers it to the workpiece. One problem with conventional contact start torches is that relative axial movement between the tip and electrode can result in alignment and axial spacing variations which adversely affect performance. As an example, many torch operators drag the tip across the workpiece as they cut. For optimum performance, it is critical to maintain distance between the tip and electrode because even small variations can compromise cut quality and speed and can also reduce the life of consumable tips and electrodes. Accordingly, there is a need for a contact start torch which can maintain the axial distance between the tip and electrode to prevent alignment and axial spacing variations.